Currently, systems (AISs) in which an automatic ship identification device for automatically transmitting and receiving data peculiar to a ship, such as a unique identification, ship's name, position, course, ship speed, and destination is mounted on each ship are employed (for example, refer to Patent Document 1). Here, in the AIS, the time-division communication system is used for communication between respective ships, and synchronization is performed on different standards for different classes. For example, between ships of CLASS A, because GPS devices are mounted on the ships due to the standard, synchronization is performed based on a 1PPS signal of the GPS, and when a GPS signal cannot be received, the synchronization is performed based on the transmission timing of the another ship having received the GPS signal. On the other hand, between ships of CLASS B′CS, a ship concerned acquires transmission timings of other ships for one minute, and the synchronization is performed based on the transmission timings of two or more other ships, which the ship concerned has continued acquiring for one minute.